Understanding and controlling the root causes of friction have long been a tireless pursuit of mankind mainly because friction impacts our safety, mobility, and environment in so many ways. Accordingly, those scientists who study friction across many scales and engineers who design, manufacture, and operate moving mechanical assemblies (MMAs), like motored vehicles, have all aimed at drastically reducing or even totally vanishing friction or achieving superlubricity at engineering scales. The superlubric regime is attractive because it would provide the highest levels of savings in energy, environment, and money. Despite the development and use of many kinds of solid and liquid lubricants in recent years, superlubricity is seldom achieved at macro or engineering scales. Generally, friction coefficients of less than 0.01 are considered superlow, and hence fall in the superlubric regime. Such levels of friction coefficients are typical of those surfaces that are either aero- or hydro-dynamically separated or magnetically levitated where little or no solid-to-solid contact takes place. Under sliding regimes where direct metal-to-metal contacts prevail and high contact pressures are present, achieving superlubric friction coefficients (i.e., less than 0.01) is difficult due to the concurrent and often very complex physical, chemical, and mechanical interactions taking place at sliding surfaces.
In theory, computer simulations, and nano-scale experiments, the feasibility of superlubricity for certain atomically smooth crystalline solids that are in dry and incommensurate sliding contacts has been demonstrated. This effect, also called structural lubricity, was theoretically predicted in 1991 and later verified experimentally between two atomically smooth sliding surfaces of single crystal silicon and graphite materials. Recently, similar observations were made between the interwalls of two nested multiwalled carbon nanotubes. To enable superlubricity, atoms in these materials are oriented in a special manner and form an atomic hill-and-valley landscape, which looks like an egg-crate. When the two graphite surfaces are in registry (every 60 degrees), the friction force is high but when the two surfaces are rotated out of registry, the friction is nearly eliminated. By way of illustration, this effect is like two egg-crates which can slide over each other more easily when they are “twisted” with respect to each other. Since this effect is due to the incommensurability of lattice planes sliding against each other, the effect is restricted to material interactions at the nanoscales. At macro-scale, this structural effect, and hence superlubricity, is lost due to the structural imperfections and disorder caused by many defects. Superlubricity is very difficult to achieve at macro-scale tribological tests and mechanical systems.
During the past decade or so, many researchers have studied graphene's properties and determined that it has outstanding thermal, chemical, optical and mechanical properties. However, very little research has been done on exploring the tribological properties, and superlubricity in particular, of graphene. Graphene exhibits high chemical inertness, extreme mechanical strength, and easy shear property on its densely packed and atomically smooth surface. Since it is ultrathin, even with multi-layers, it may be applied onto MEMS/NEMS devices for operation and use at the oscillating, rotating and sliding contacts to reduce friction and wear.
Solid lubricants, such as, graphite, hexagonal boron nitride, molybdenum disulfide, and boric acid are traditionally used to combat friction and wear in a variety of moving mechanical assemblies. These lubricants may be applied as thin or thick solid coatings or burnished and sprayed onto surfaces to achieve lubricity. The durability or lifetime remains an important issue when long wear life is desired. In short, an important quandary with these pre-existing solid lubricants is how long the solid lubricant coatings will last. Additionally, in some specialty applications such as protective coatings for magnetic disc drives, in addition to low wear and friction characteristics, an extremely thin coating (down to nm or less) is an important.
Recent advances in the synthesis of graphene have resulted in several studies demonstrating the unique nanomechanical and nanotribological properties of single and few layer graphene. While some studies have recently demonstrated the potential of multi-layer graphene in pure or in additive form in substantially reducing wear and friction in mechanical systems, there are no reports on the tribological behavior of single or few layer graphene at macroscale.